A typical radiation image sensing apparatus that has conventionally been used is an apparatus that combines a photosensor having a MIS-TFT structure constructed by a MIS photoelectric conversion element and a switching TFT and a phosphor to convert radiation into visible light. In this specification, radiation includes not only α-rays, β-rays, and γ-rays but also electromagnetic waves such as visible light and X-rays.
FIG. 9 is an equivalent circuit diagram showing the circuit arrangement of a conventional radiation image sensing apparatus. FIG. 10 is a plan view showing the layout structure in the conventional radiation image sensing apparatus.
As an example of a radiation image sensing apparatus, one photoelectric conversion element (semiconductor conversion element) and one thin film transistor (TFT) are arranged for each pixel. More specifically, a pixel on the ath row and bth column from the upper side in FIGS. 9 and 10 has one photoelectric conversion element Mba and one thin film transistor Tba (a, b=1, 2, 3, 4).
Four photoelectric conversion elements arranged on the bth column are connected to a common bias line Vsb so that a predetermined bias is applied from a reading unit. The gate electrodes of four TFTs arranged on the ath row are connected to a common gate line Vga so that the gates are ON/OFF-controlled by a gate driving unit. The source electrodes or drain electrodes of the four TFTs arranged on the bth column are connected to a common signal line Sigb. Signal lines Sig1 to Sig4 are connected to the reading unit.
A phosphor layer that converts X-rays into visible light is formed on the irradiation surface of the radiation image sensing apparatus.
X-rays that irradiate an object such as a human body to be inspected on the radiation image sensing apparatus pass through the object to be inspected while being attenuated by it. The X-rays are converted into visible light by the phosphor layer. The visible light strikes the photoelectric conversion element and is converted into charges. The charges are transferred to a signal line through TFTs in accordance with a gate driving pulse applied from the gate driving unit and output to the outside through the reading unit. After that, charges that are generated by the photoelectric conversion element and remain there without being transferred are removed through the common bias line. This operation is called “refresh”.
FIG. 11 is a sectional view showing the layer structure of one pixel of a photosensor having a conventional MIS-TFT structure. FIG. 11 shows a photosensor in which a MIS photoelectric conversion element and a switching TFT are formed in parallel.
An MIS photoelectric conversion element 1001 and switching TFT 1002 are formed on an insulating substrate 1011. The MIS photoelectric conversion element 1001 has a lower electrode 1017, insulating layer 1018, semiconductor layer 1019, n+-semiconductor layer 1020, and upper electrode 1022. The switching TFT 1002 has a gate electrode 1012 gate insulating layer 1013, semiconductor layer 1014, ohmic contact layer 1015, and two, source and drain electrodes 1016.
The lower electrode 1017 and gate electrode 1012 are formed from the same electrode layer. The insulating layer 1018 and gate insulating layer 1013 are formed from the same insulating layer. The semiconductor layer 1019 and semiconductor layer 1014 are formed from the same semiconductor layer. The upper electrode 1022 and source and drain electrodes 1016 are formed from the same electrode layer.
The lower electrode 1017 of the MIS photoelectric conversion element 1001 is connected to one of the source and drain electrodes 1016 of the switching TFT 1002. The upper electrode 1022 is connected to a bias line. The other of the source and drain electrodes 1016 is connected to a signal line. The gate electrode 1012 is connected to a gate line. An insulating layer (protective layer) 1025, organic protective layer 1026, adhesive layer 1027, and phosphor layer 1028 are formed on the elements.
An X-ray automatic exposure controller (AEC) which automatically controls exposure of X-rays emitted from an X-ray source in the radiation image sensing apparatus will be described next.
Generally, in a radiation image sensing apparatus having two-dimensionally arrayed sensors, the dose of incident X-rays must be adjusted (AEC-controlled) for each object to be inspected or every imaging. X-ray dose adjustment methods can be classified into two methods.
(1) An AEC sensor is arranged independently of the radiation image sensing apparatus.
(2) An X-ray dose is read out from all or some of the image sensors in the radiation image sensing apparatus at a high speed, and the read signal is used as an AEC signal.
Conventionally, when the method (1) is employed, a plurality of thin AEC sensors which attenuate X-rays by about 5% are separately arranged in front of the radiation image sensing apparatus, i.e., on the detected object side of the phosphor layer of the radiation image sensing apparatus. X-ray exposure is stopped on the basis of the outputs from these AEC sensors, thereby obtaining an appropriate X-ray dose for imaging. As an AEC sensor used in this method, a sensor which directly extracts X-rays as charges by using an ion chamber, or a sensor which extracts phosphor light through a phosphor by using a fiber and causes a photomultiplier to convert the light into charges is used.
However, when AEC sensors are separately prepared in the radiation image sensing apparatus in which sensors are two-dimensionally arrayed to adjust (AEC-control) an incident radiation dose, the layout of the sensors poses a problem.
Generally, information necessary for AEC is present at the center of an object. If AEC sensors should be laid out without impeding image sensing by image sensing sensors, AEC sensors that attenuate radiation by only a minimum amount must be independently arranged, resulting in an increase in cost of the entire apparatus. In addition, there are no sensors that do not attenuate radiation at all. Hence, the quality of a sensed image inevitably degrades.
The method that uses image sensing sensors in the radiation image sensing apparatus as AEC sensors poses no serious problem for sensors with a relatively small number of pixels. However, when the number of pixels is, e.g., 2,000×2,000, a high-speed driving circuit is necessary, resulting in an increase in cost of the entire apparatus. Since high-speed driving is necessary, it is difficult to sufficiently ensure the charge storage time, charge transfer time, and capacitor reset time in the image sensing sensors. As a result, the quality of a sensed image degrades.
Contrary to this arrangement, U.S. Pat. No. 5,448,613 discloses an arrangement in which a second pixel group is arranged in a sensor substrate and driven by a shift register different from that for an image read sensor to detect the integration of signal charges.
However, when this arrangement is simply employed, some of image read pixels are replaced with second pixels. Accordingly, the opening ratio of pixels related to image reading with respect to all the pixels decreases. In addition, lead interconnections must be prepared separately for the first pixels and second pixels. This may complicate the interconnection structure.
Hence, there is still room for improvement in the arrangement of the above prior art in association with the pixel layout and interconnection structure.